What is alkalinity
Alkalinity is a fundamental concept in geochemistry, particularly aquatic and marine chemistry, representing the ability of a solution to resist acidification. It serves as a measure of the solution’s acid-neutralizing capacity (ANC).
alkalinity
Charge Balance Definition:
- Alkalinity is defined as the charge deficit between the sum of dissolved conservative cations (e.g., Na⁺, K⁺, Ca²⁺, Mg²⁺ and conservative anions such as Cl⁻, SO₄²⁻, NO₃⁻) in an electrolyte solution.
- Conservative ions are those whose concentrations are not significantly affected by changes in pH
- The result of this charge deficit must be balanced by a combination of non-conservative anions.
- The total numerical value for Total Alkalinity (A_T) is equal to the sum of conservative cation charges minus the sum of conservative anion charges.
alkalinity
Conservative ions
are those whose concentrations are not significantly affected by changes in pH.
alkalinity
Proton Balance Definition
- Alkalinity is the excess in bases (proton acceptors) over acids (proton donors) in the solution.
- The missing constraint needed to uniquely solve the system of acid/base equations in seawater is the expression for A_T, which represents the charge balance.
alkalinity
Operational Definition (Titration)
Alkalinity is defined as the total quantity of acid that must be added to the solution to bring the pH to the CO₂ equivalence point. At this point (pH 4.5), nearly all proton acceptors (CO₃²⁻ and HCO₃⁻) have been converted to H₂CO₃. The units of alkalinity are typically equivalents per kilogram (eq kg⁻¹) or milliequivalents (meq) per kilogram.
Components of Alkalinity
Dominant Components
Alkalinity in marine waters is dominated by the carbonate system (bicarbonate and carbonate ions) and borate species (B(OH)₄⁻). These two groups define about 99% of the total alkalinity in surface seawater.
Components of Alkalinity
Carbonate alkalinity (AC or cALK)
This term includes only the contributions from bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻):
AC = [HCO₃⁻] + 2[CO₃²⁻]
The carbonate ion (CO₃²⁻) has a coefficient of 2 because it carries a double negative charge and can accept two protons before becoming carbonic acid (H₂CO₃). Carbonate alkalinity typically makes up about 96% of the total alkalinity.
Components of Alkalinity
Total Alkalinity (AT)
The complete definition of total alkalinity for oxic seawater includes all known inorganic proton acceptors and donors, referenced to the CO₂ equivalence point:
Aₜ = [HCO₃⁻] + 2[CO₃²⁻] + [B(OH)₄⁻] + [OH⁻] + [H₃SiO₄⁻] + [HPO₄²⁻] + 2[PO₄³⁻] + … − [H⁺] − [HSO₄⁻] − …
In surface seawater where nutrient concentrations are low, silicate and phosphate contributions are often negligible, typically contributing less than 0.2% of the total alkalinity.
alkalinity and its Role in the Carbonate System and Geological Processes
Buffer System
Alkalinity is essential for the buffering capacity of seawater, which stabilizes the pH of the ocean within a narrow range (7.6 to 8.2 in surface waters). This buffering capacity results from the partial dissociation of weak acids like carbonic acid H₂CO₃
Relationship to DIC and CO₃²⁻
Alkalinity and Dissolved Inorganic Carbon
DIC = [H₂CO₃] + [HCO₃⁻] + [CO₃²⁻]
are the two parameters most frequently measured and used to calculate the pH of seawater and the concentration of the carbonate ion CO₃²⁻
The difference cALC-DIC if CO2 is ignored gives the carbonate ion concentration CO₃²⁻
Conservative Property:
Alkalinity is generally considered a conservative quantity. However, in an oceanographic context, it is not conservative because biological processes, particularly the precipitation and dissolution of calcium carbonate CaCO, significantly modify alkalinity within the water column.
Geological Control:
weathering
The imbalance of non-protonating ions (which defines alkalinity) is caused by the chemical weathering of rocks. The chemical erosion of silicates creates fresh alkalinity (e.g., the weathering of calcic plagioclase into kaolinite and dissolved ions like Ca^2+ and HCO3^-
geological control
CaCO3 cycling
- The production and dissolution of biogenic carbonate (CaCO3) are the main cause of the systematic depth variation in alkalinity in the oceans.
- When one mole of CaCO3 dissolves, it causes an increase in alkalinity that is twice the increase in DIC.
- Conversely, precipitation of CaCO3 removes alkalinity and reduces pH.
aquifer
What is an aquifer
An aquifer is a geological formation or a subsystem of the Earth’s environment that serves as a reservoir for groundwater, which is typically derived from atmospheric precipitation.
aquifer, Structure and Composition
Water Storage:
Groundwater, the water stored within an aquifer, is the second largest reservoir of liquid water on Earth. Most of the available freshwater (0.5% of all water) is present in aquifers.
aquifer, Structure and Composition
Location and Material:
Water within an aquifer is usually present in the interstitial pore spaces of rocks and soil or in fractures to considerable depths, even on the continents. Aquifers can be composed of various materials, such as limestone, dolomite, granitic glacial sand, porous sandstone, or carbonate systems.
aquifer, Structure and Composition
Recharge
Water from precipitation, known as meteoric water, infiltrates and percolates into the ground until it reaches an impermeable layer, forming the aquifer. In coastal areas, there may also be a component of seawater present.
aquifer
Chemical Interaction
- Water in an aquifer is not pure but is a solution formed by interaction with minerals. - - Hydrogeologists commonly trace the chemical history of water from an aquifer by studying the relative abundances of dissolved ions.
**Buffering: **Ion exchange reactions involving colloidal clay particles play a key part in regulating the composition of natural waters and buffering the major element composition of the groundwater in an aquifer.
Mineral Deposition: Convecting pore fluid circulating through a sandstone aquifer can deposit minerals like quartz or calcite, depending on temperature gradients and solubility changes. The precipitation of uranium ore in sandstone roll-front deposits occurs in the mixing zone interface between an oxidized, uranium-bearing meteoric fluid and the reduced portion of the aquifer.
**Salinity: **Deep basinal waters (connate fluids) trapped in deeply buried sedimentary rocks can become highly saline (brines) due to long-term contact with host lithologies at elevated temperatures.
aquifer
Aquifers as Geochemical Systems
Closed System: An aquifer may be considered a closed system if the confining beds have an acceptably low permeability and the study is focused only on relatively rapid chemical reactions within it.
Open System: An aquifer might more realistically be considered an open system if long-term issues, such as toxic waste disposal, or processes with slow transfer rates across its boundaries are considered.
The integrity of an aquifer is crucial, as is demonstrated by the practice of determining the age of water in underground aquifers using 36Cl to ensure that the withdrawal rate does not exceed the recharge rate. For example, the extensive use of water from the Ogallala Aquifer has led to groundwater overdraft in certain regions.
What is the water constant
The term “water constant” generally refers to the dissociation constant of water, symbolized as KW (or K(H2O)in some contexts), which quantifies the equilibrium of water’s self-ionization (autodissociation).
self-ionization of water
The reaction in which water acts as both a proton donor and a proton acceptor is called the autoionization (self-ionization) of water.
Water is amphoteric, meaning it can behave as both a Brønsted–Lowry acid (a proton donor) and a Brønsted–Lowry base (a proton acceptor).
There are two equivalent ways to write the reaction:
- H₂O ⇄ H⁺ + OH⁻
(This is a simplified representation. Free H⁺ does not actually exist in solution.) - 2H₂O (liquid) ⇄ H₃O⁺ (aqueous) + OH⁻ (aqueous)
(In this form, one water molecule donates a proton and the other accepts it.)
Even in pure water, a very small fraction of water molecules transfer protons between each other, producing hydronium and hydroxide ions.
The ion-product constant for water
Kw and is the equilibrium constant for the reaction:
Kw = (H⁺)(OH⁻)
Standard Value:
In pure water at 25 °C (about 298.15 K) and standard pressure, Kw has a value of 10⁻¹⁴. Because Kw is 10⁻¹⁴, the activities (or, in pure water, the effective concentrations) of H⁺ and OH⁻ ions are each about 10⁻⁷ mol kg⁻¹.
Logarithmic Notation:
This constant is often written in negative-log form as pKw, similar to pH. At 25 °C, pKw = 14.
water constance
temp dependence
Like all equilibrium constants, the value of Kw depends on temperature.
For pure water, the dissociation constant Kw (often written as Kw at ionic strength zero) increases as temperature increases.
The reference material also gives equations for calculating the natural logarithm of Kw in seawater (written as K′w), which depends on both temperature (T in Kelvin) and salinity (S). One example of such an equation for the dissociation constant of water in seawater, expressed in mol² per kilogram, is:
ln Kw = 148.96502 − (13847.26 / T) − 23.65218 × ln(T)
+ [(118.67 / T) − 5.977 + 1.0495 × ln(T)] × S^(1/2)
− 0.01615 × S
The temperature dependence of Kw is also commonly presented in tables showing how its value changes at different temperatures.
water constant
role in water chemistry
Here is the same content written clearly in plain text (and with the small typo corrected):
The water constant is essential for describing acid–base equilibria in natural waters.
pH Scale:
It defines the neutral point of the pH scale. The pH of pure water is 7.0.
Solvated Species:
Although many thermodynamic treatments use H⁺ as the proton product, in reality most protons associate with water molecules to form the hydronium ion, H₃O⁺. These hydronium ions then interact with additional water molecules through hydrogen bonding.
Charge Balance:
In the carbonate system, the water constant provides a necessary equilibrium relationship that links the concentrations of H⁺ and OH⁻, allowing the full system of acid–base equations to be solved.
